The invention is a process and apparatus for dehydrating gas, such as natural gas. The process uses an absorption system to dehydrate the gas, and a membrane pervaporation unit to regenerate the water-laden desiccant.
Natural gas as obtained from the well contains water vapor. Before the gas can be passed to the pipeline, it must be dried to prevent problems such as hydrate or ice formation or corrosion.
Glycol dehydration, which is simple and inexpensive, is currently the most widely used method of dehydrating natural gas. The estimated number of glycol dehydration systems operating in the United States alone is at least 55,000.
In a typical dehydrator, wet gas is scrubbed with dry glycol to yield a product gas of lowered water dew point. The glycol absorbs not only water, but also aromatic compounds, such as BTEX, and other hydrocarbon vapors.
The water-rich or xe2x80x9cspentxe2x80x9d glycol is passed to a regeneration system, which typically includes a flash tank, where methane and other light gases are flashed off, a reboiler and a regeneration still column. The water-laden glycol is heated to drive off absorbed water, and dry glycol is recovered for reuse in the absorber. Unfortunately, this heating also vaporizes hydrocarbons that have been sorbed into the glycol, and these are expelled, along with the water vapor, in the hot overhead vent stream from the still column.
Although the major component is steam, this overhead vent stream may contain as much as 20 mol % or more of organic compounds, including aromatic and non-aromatic organic vapors. Of these organic compounds, a significant proportion may be the aromatic compounds benzene, toluene, ethylbenzene and xylene, together commonly known as BTEX compounds. These organic emissions are now classified as Hazardous Air Pollutants (HAPs), and are subject to emissions regulations both in the United States and internationally.
There is, therefore, a need for a simple, reliable and cost-effective method to reduce or eliminate the release of these organic components.
So-called xe2x80x9cenhancedxe2x80x9d regeneration systems have been used to increase water removal from the spent glycol, and some of these systems can reduce HAP emissions. For example, U.S. Pat. No. 5,141,536, to Texaco, describes an apparatus with a condenser unit in the reboiler overhead vent to condense hydrocarbon components and some water, thus venting a hydrocarbon-depleted water vapor stream. U.S. Pat. No. 5,234,552 describes a condensation system to recover liquid hydrocarbons and collect remaining hydrocarbon vapors for use as fuel for the glycol reboiler. U.S. Pat. No. 5,788,745, to Phillips Petroleum, describes the use of both a condenser and a phase separator to recover liquid hydrocarbons.
Another technique for glycol regeneration is stripping against a gas or steam. A typical configuration for a stripper is a packed or multi-tray tower, with the stripping gas normally flowing upward countercurrent to the descending liquid glycol. Depending on the stripping agent used, water, hydrocarbons, or both are absorbed from the glycol into the stripping gas, thus regenerating the glycol for reuse in dehydrating the natural gas.
U.S. Pat. No. 5,643,421, to OPC Engineering, describes a glycol regeneration process comprising flash evaporation to recover hydrocarbon components from the spent glycol, followed by heating and multiple stripping steps using a vaporized solvent as the stripping agent to dehydrate the glycol. U.S. Pat. No. 5,766,423, also to OPC Engineering, uses basically the same process, but includes condensation of a portion of the once-stripped glycol stream for additional removal of hydrocarbons.
U.S. Pat. No. 5,490,873, to Bryan Research and Engineering, combines condensation of the reboiler overhead with further treatment of the lean glycol stream by stripping against a sidestream of the dehydrated natural gas. U.S. Pat. No. 5,453,114, to Ebeling, uses a stripping step to remove hydrocarbons upstream of the glycol reboiler. U.S. Pat. No. 5,209,762, to Gas Research Institute, uses an integrated combination of condensation and steam stripping to produce dischargeable water, organic liquid and an organic vent gas that may be useful as fuel.
U.S. Pat. No. 5,725,636, also to Gas Research Institute, adds potassium salts to the glycol to increase water absorption and decrease hydrocarbon absorption in the natural gas dehydration step. Since hydrocarbons are less readily absorbed, the glycol has greater water-absorbing capacity. The regeneration process also produces a leaner glycol stream and the water vapor vented to the atmosphere has a lower hydrocarbon content.
These and other processes can achieve low water concentrations in the glycol, which in turn provides good dehydration of the raw gas, resulting in low dew points in the treated gas. They can also reduce, but do not eliminate, BTEX and other HAP air emissions through the additional treatment step. However, most of these processes produce an additional gaseous or aqueous waste stream that requires on-site or off-site attention such as incineration, disposal, or further treatment.
Despite these efforts, a cost-effective regeneration technology that truly minimizes or eliminates HAP emissions has not been developed. There yet remains a need for such a process.
Membranes have for many years been known to be usable in pervaporation mode for dehydrating organic liquids. For example, many patents to GFT and others, such as U.S. Pat. Nos. 3,720,717, 4,405,409 and 4,755,299, describe such separations using polyvinyl alcohol (PVA) membranes.
A large number of patents to Texaco also disclose pervaporation processes using membranes of various types for dehydrating organic liquids, such as organic acids. In particular, U.S. Pat. No. 4,802,988 describes PVA membranes and processes for treating ethylene glycol/water solutions to reduce the ethylene glycol content in the permeate to low levels, and U.S. Pat. No. 5,182,022 describes a similar process using ion-exchange type sulfonated polyethylene membranes to produce a low-glycol permeate.
U.S. Pat. No. 5,552,023, to Allied Signal, describes a process to treat spent deicing fluid (ethylene glycol mixture) using a combination of membrane distillation through a porous polytetrafluoroethylene membrane and reverse osmosis. U.S. Pat. No. 5,554,286, to Mitsui Engineering, describes an A-zeolite-type membrane for pervaporation applications, and includes sample data for dehydration of alcohols.
U.S. Pat. No. 5,350,519, to Membrane Technology and Research, describes a process including a condenser to condense the glycol reboiler overhead vent stream and recover liquid hydrocarbon components, followed by a pervaporation step of the waste aqueous stream to recover additional hydrocarbon components and vent a hydrocarbon-depleted water stream.
The invention is a process and apparatus for dehydrating gas, especially, but not necessarily, natural gas. The process uses absorption into a liquid desiccant to dehydrate the gas, and pervaporation to regenerate the water-laden desiccant. The pervaporation step can both regenerate the desiccant and capture any hazardous organic components that may be present in a single step.
In a basic embodiment, the process of the invention includes the following steps:
(a) subjecting a gas stream containing water to an absorption step, comprising the steps of:
(i) contacting the gas stream with a liquid desiccant in an absorber;
(ii) withdrawing from the absorber a dehydrated gas stream;
(iii) withdrawing from the absorber a spent desiccant stream comprising desiccant and water; and
(b) subjecting the spent desiccant to a regeneration step, comprising:
(i) providing a membrane unit containing a membrane having a feed side and a permeate side and exhibiting a pervaporation separation factor in favor of water over the desiccant;
(ii) passing the spent desiccant into the membrane unit and across the feed side under pervaporation conditions;
(iii) providing a driving force for transmembrane permeation;
(iv) withdrawing from the feed side a residue desiccant stream depleted in water compared with the spent desiccant stream;
(v) withdrawing from the permeate side a permeate stream comprising water vapor.
The most common desiccants are glycols, particularly ethylene glycols.
The absorption step may be performed in any convenient manner that brings gas and desiccant into good partitioning contact. Typically, but not necessarily, it is carried out in an absorption column or tower by flowing the gas stream to be dried upward in counter-current contact with the down-flowing liquid desiccant. In this case, the drier gas stream is withdrawn from the top of the column and the water-laden liquid sorbent is withdrawn from the bottom. As just one alternative to the traditional tower, a membrane contactor can be used for the absorption step.
The reduction in water dew point of the gas achieved in the absorption step depends on the sorbent and the operating conditions. Dew point reductions of at least about 40xc2x0 C. or more can normally be accomplished, and much greater reductions, such as 70xc2x0 C. or more, are possible in many cases.
The membrane separation step is carried out in pervaporation mode. That is, the membrane feed stream is in the liquid phase, the permeate is in the gas phase, and the driving force is provided by maintaining a permeate partial pressure lower than the feed saturation vapor pressure for permeating components.
This partial pressure difference may be provided in any convenient manner, such as by one or more of heating the feed, cooling the permeate, drawing a partial vacuum on the permeate side, and providing a gas sweep on the permeate side. It is preferred either simply to heat the feed liquid, thus raising the water vapor pressure on the feed side of the membrane, or to both heat the feed and use a sweep gas on the permeate side.
The membrane separation unit may contain any type of membrane capable of selectively passing water and rejecting the desiccant when operated in pervaporation mode. For example, polymeric membranes or inorganic membranes may be used. A most preferred membrane is a composite membrane having a ceramic support coated with a selective layer of a zeolite or silica.
The water concentration in the desiccant stream that forms the membrane feed may be any value. However, the water content will normally be substantially less than 50 wt %, and usually will be no more than about 20 wt %, and typically might be between about 5 wt % and 10 wt %.
The water concentration in the membrane residue stream should preferably be sufficiently low that the desiccant can be reused in the absorption step without additional water removal treatment. Depending on the application, this usually means that the water content of the residue stream should be below 5 wt %, and typically much lower, for example below 2 wt %, 1 wt %, 0.5 wt % or less.
The process is particularly useful for treating natural gas using glycol as the desiccant. In a typical glycol absorption process, lean glycol is contacted with natural gas in a counter-current column. The glycol absorbs water from the wet gas. Organic compounds that are also present in the gas, such as aliphatic hydrocarbons, and especially BTEX and other aromatic hydrocarbons, are soluble in the glycol sorbent and are extracted with the water.
The spent glycol, laden with water and containing the sorbed organic components, is heated and sent to the membrane separation unit, where it is passed across the surface of a membrane with a high separation factor in favor of water over organic compounds such as glycol and BTEX hydrocarbons. The vapor permeating the membrane is sufficiently free of such organic components to meet requirements for discharge to the environment.
The residue stream, containing lean glycol, the BTEX aromatics and any other hydrocarbon components, may be recirculated to the absorption column.
Thus, in this typical, preferred embodiment relating to natural gas, the process of the invention includes the following steps:
(a) subjecting a natural gas stream to an absorption step, comprising the steps of:
(i) contacting the natural gas stream with glycol in an absorber;
(ii) withdrawing from the absorber a dehydrated natural gas stream;
(iii) withdrawing from the absorber a water-laden glycol stream; and
(b) subjecting the water-laden glycol stream to a regeneration step, comprising:
(i) providing a membrane unit containing a membrane having a feed side and a permeate side and exhibiting a first pervaporation separation factor in favor of water over glycol and a second pervaporation separation factor in favor of water over benzene;
(ii) heating the water-laden glycol stream;
(iii) passing the heated water-laden glycol stream into the membrane unit and across the feed side under pervaporation conditions;
(iv) withdrawing from the feed side a regenerated glycol stream;
(v) withdrawing from the permeate side a permeate stream comprising water vapor;
(vi) recirculating at least a portion of the regenerated glycol stream to the absorption step.
In this case, the spent glycol that forms the membrane feed stream is typically heated to a temperature between about 150xc2x0 C. and 250xc2x0 C., and the preferred membrane type is an inorganic membrane, such as a silica membrane, that can operate without difficulty at high temperatures.
By operating at such a feed temperature, and optionally passing a sweep gas, such as air, to lower the partial pressure of water vapor on the permeate side, it is possible to reduce the water content of the regenerated glycol to a low level, such as 1 wt %, 0.5 wt %, 0.2 wt % or even less. Thus, the glycol that is returned to the absorption column is drier than is the case when glycol is regenerated in a conventional reboiler, which typically reduces the water content to no lower than about 2 wt %.
The use of drier glycol in the absorption step means that the dew point of the treated natural gas can be reduced compared with conventional dehydration. The dehydrated gas may have a water dew point as low as xe2x88x9220xc2x0 C., xe2x88x9230xc2x0 C. or lower, for example.
Expressed as a reduction in dew point, the process of the invention is able to achieve reductions of water vapor dew point of as much as 50xc2x0 C., 60xc2x0 C., 70xc2x0 C., 80xc2x0 C. or even more.
The membranes provide high separation factors in favor of water over all organic components. Thus, the BTEX concentration in the water vapor permeate stream is generally well below 1,000 ppmv, and typically no more than about 100 ppmv, and the glycol content is negligibly small, such as less than 50 ppmv. These low levels enable dehydrators to operate so as to meet various requirements and regulations in many regions for HAPs emissions. Thus, in general, the permeate stream may be safely discharged to the atmosphere.
In a final aspect, the invention is apparatus for carrying out a dehydration process, and comprising the following elements:
(a) an absorber, having a wet gas inlet, a dehydrated gas outlet, a liquid desiccant inlet and a spent liquid desiccant outlet, and being adapted to bring wet gas into water-partitioning contact with the liquid desiccant;
(b) a membrane separation unit, having a feed inlet, a feed outlet, and a permeate outlet, and containing an inorganic membrane having a feed side and a permeate side, and characterized by the ability to provide pervaporation separation factors of at least about 100 in favor of water over desiccant and in favor of water over benzene when used to treat a liquid desiccant stream containing about 5 wt % water and 1 wt % benzene at 200xc2x0 C.;
(c) first connection means connecting the membrane separation unit feed inlet and the absorber spent liquid desiccant outlet, so that spent liquid desiccant may pass out of the absorber and into the membrane separation unit;
(d) second connection means connecting the membrane separation unit feed outlet and the liquid desiccant inlet, so that regenerated liquid desiccant may pass out of the membrane separation unit and into the absorber;
(e) heating means positioned between the absorber and the membrane separation unit to enable spent liquid desiccant to be heated before entering the membrane separation unit.